DOT3 or DOT5 Brake fluid

S

Stoker280

Junior Member
I recently replaced the entire brake system on my 1953 Mercury M250 Classic. I am looking for some advice on whether to use DOT3 or DOT5 fluid. I will not be driving the vehicle a lot and it will more then likely sit for over 6 months per year. Any advice helps as I am new to the restoration game.
 
I use dot 5 also, but it will eat pressure brake switches, even those rated for dot 5. Switch over to a brake pedal switch and you'll be better off anyway.
 
You have the ideal situation (new system) to install DOT5.
Did the same thing 25 years ago with no regrets.
Only change was a mechanical brake light switch.
 
I have used DOT5 fluid in my '48 F-1 for about 10 years with no problems. I also use DOT3 in all the rest of my cars with no problems. So, who knows?
 
I put all five new cylinders in the brake system of my F1 in the year 2000 and used DOT5 and no problems since.
The pickup sits out in the open all the time too.
Do not use DOT5 unless you have new cylinders.
That stuff will leak where other fluids won’t.
I use a mechanical brake light switch for a fifties Chev. It goes under the brake pedal.
 
Cars don't like long slumbers. I'd use DOT 3 as long as the car is used a lot. DOT 5 turns yellow to amber color within a year and I've had moisture problems with it as well as DOT 3. Either one will work but I don't really see that one is any better than the other over a long period of time.
 
The fact that DOT5 does not work with anti-lock brakes (which are almost universally used these days) means it will probably get harder to get and more expensive in the future.

Something to think about.
 
I've had dot-5 in my 50 for about 10 years now. The only advice I would pass on is: When you bleed the system, pump the brake pedal very slowly. If you pump it too fast you will get air bubbles in the fluid and they are almost impossible to get rid of causing a soft brake pedal.
 
I did a Corvette about 20 years ago, and the instructions that came with the DOT5 fluid said to heat it to almost the boiling point to drive the air out of it. I bought a cheap teakettle and heated it on the stove in my shop. I was very careful not to jiggle it when carrying it to the car, and I have to say that the teakettle made filling the master cylinder quick and easy. Being very careful with everything, I was finally able to get the brakes bled successfully, and they worked perfectly until I sold the car 3 years ago.

The bottom line was that, between the extra expense and extra effort it took to get the Corvette right, I never used it again. I was cured.
 
If you're thinking of using dot3 and not dot5 then in my experience dot 4 is better. I did some testing on clutch slave cylinders years ago and the seals lasted longer with dot 4.

Mart.
 
DOT 5 is a silicone based fluid, and is not compatible with DOT 3... so the system needs to be COMPLETELY flushed in order to make the switch.

DOT 4 is basically Synthetic DOT 3, and is compatible. That's what I would probably use if the system already had DOT 3 in it.

There is also DOT 5.1 which I don't know a lot about... so I'm not going to pretend and give you bad advice in regards to. lol
 
I did DOT 5 in my 51-yr-old pickup, about in the 1980's. It (DOT5) has been ok, and especially ok, as my old pickup sits out in the weather. I opened up the system about 15 years ago, basically for service and to see what all the parts looked like. Everything still looked clean and pristine. So... It's been ok. But, at this stage of my life, I'd never bother with it again. The DOT 3 and 4 suggestions are what I've followed in all recent years. I, for one, am not messing with 5 again.
edit... I should have added, on my DOT3/4 cars, I flush the system, every few years, based on accumulated moisture. I have done yearly flushes, which seems to be more often than necessary. Then I switched to flushing every two or three years, and that seems to be ok. A primary interest for me, on modern cars, is to keep all the abs stuff in good working order.
 
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I agree with what "bobH" said above. I have mentioned this before, but I purchased a relatively cheap tester that tells you the moisture content of DOT3/4 brake fluid. You don't have to change it until it reaches a certain level. I have found that, with cars stored inside, it takes about 9 to 15 years for the fluid to reach that moisture level. I check my cars every spring.
 
FL&WVMIKE said:
DOT 5 is a silicone based fluid, and is not compatible with DOT 3... so the system needs to be COMPLETELY flushed in order to make the switch.

DOT 4 is basically Synthetic DOT 3, and is compatible. That's what I would probably use if the system already had DOT 3 in it.

There is also DOT 5.1 which I don't know a lot about... so I'm not going to pretend and give you bad advice in regards to. lol
Click to expand...

I agree with everything said above. I would just add that DOT 5 can have a spongier pedal than DOT 3, 4 or 5.1.

Bruce
 
It's just fluid dynamics. Hydraulics. Viscous fluid. Some say mixing on accident will ruin brake systems.
 
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